How to Grow and Care for Sea Holly

Sea holly is a low-maintenance, long-lived perennial in the carrot plant family (Apiaceae). Its dark green leaves produce taller flower stems with striking purple-blue flowers. The flowers look like small glowing thistles and often bloom from midsummer until fall.

Sea holly grows best in dry to medium sandy soil in a location that boasts full sun, needing at least eight hours of sunlight. It's hardy in USDA zones 5 to 9, down to -20 degrees Fahrenheit. Once established, sea holly only needs water when it's dry since it has a long taproot that makes it drought-resistant. Sea holly is self-sowing but non-invasive since it can be easily removed.

Sea Holly Care

Sea holly plants are easy to care for if you provide the proper soil and sun conditions. You'll want to plant sea holly in dry to medium sandy soil in a location that boasts full sun. Once planted, it will require little tending. For most varieties, flowering starts in midsummer and will continue well into the fall.

Here are the main care requirements for growing a sea holly:

• Plant in a full sun location in a forever spot; its long taproot makes it difficult to transplant.
• Tolerates irregular water; drought resistant.
• Prefers sandy, well-drained soil; salt tolerant.
• Needs support when grows tall; plant next to other tall plants, like coneflowers.
• Handles fluctuations in temperature and humidity well in USDA zones of 5 to 9.
• Does not need fertilizer and handles poor soil.

Light

A full day of sun (at least eight hours) will give you the strongest sea holly plants and the most blooms. While the plants can handle partial shade, the reduction in light may lead to weaker stems, which may make it necessary to stake the plants to keep them upright.

Soil

The best planting medium for sea holly plants is well-drained, sandy, poor to moderately fertile soil. It has a good tolerance for salty soil and does well in coastal gardens. Soil that is too rich can make sea hollies sprawl. Good drainage is a must, so amend compact soil with compost.

Sea holly is not particular about its soil pH—anything around the neutral range is sufficient (pH 6.1–7.8). However, the plants need good drainage or will be susceptible to root rot and can die off.

Water

Thanks to the long taproot, sea holly plants are drought tolerant once established and won't need additional water unless subjected to a prolonged drought at the peak of summer. Excess surface moisture can cause the flower's crown to rot, so sea holly plants should be somewhat segregated from other plants that require more water.

When watering, aim the water source at the base of the plant to reduce the risk of built-up moisture.

Temperature and Humidity

As long as it's planted within its proper hardiness range (zones 5 to 9), sea holly has no additional temperature or humidity requirements. It will do better in the cooler days of spring and fall but will not die off in warmer weather.

Fertilizer

Sea holly plants are not heavy feeders. Too much fertilizer will make the plants sprawl. Plant these where you have poor, moderately fertile soil in your garden.

Types of Sea Holly

Here are some favorite sea holly varieties:

• 'Big Blue' has iridescent, spiky 4-inch blue blooms and silvery foliage; it grows 18 to 24 inches tall and wide.
• 'Jade Frost' is a variety that boasts variegated foliage with pink margins and veins.
• 'Blue Glitter' is a bountiful variety with dozens of blooms per plant and gray-blue foliage.
• 'Sapphire Blue' is a classic favorite with striking blue flowers and leaves.
• 'Tiny Jackpot' is a petite variety that grows to just 14 inches tall and looks good at the front of a border.

Other Types of Eryngium Plants

Eryngium species have historically been used in traditional medicine for treating kidney issues. Research shows that Eryngiam has antioxidant and anti-inflammatory properties. Non-toxic to humans, the shoots and leaves of Eryngium species, including sea holly, are edible in small quantities.

The genus has more than 150 species; several are common garden plants. All have similar metallic blue flowers, though the size of the blossoms varies somewhat. Some common species include:

• E. alpinum (alpine sea holly) is a 30-inch tall plant, hardy in zones 2 to 8.
• E. giganteum (giant sea holly) grows to 4 feet tall and is hardy in zones 4 to 7.
• E. variifolium (Moroccan sea holly) is an 18-inch tall plant, hardy in zones 5 to 9.
• E. bourgatii (Mediterranean sea holly) has smaller stature (24 inches) and smaller flowers. It is hardy in zones 5 to 9.

Pruning

Sea holly will bloom longer if you deadhead the spent flowers. Prune the plants off near ground level at the end of the growing season. But the flowers also look wonderful long into winter, so you always have the option to leave the fall flowers on the stems.

Propagating Sea Holly

Spring is the best time to plant mature plants—sea holly will grow quickly, and semi-mature nursery plants will flower in their first season. Sow seeds in late summer or fall, and they'll germinate in spring.

Sea holly's taproot can make it a bit difficult to propagate, but it's possible to do it via root cuttings. Here's how:

1. Carefully dig up the plants in late summer (taproot and all), then cut away healthy sections of roots (but never cut away more than one-third of the plant's total root mass)
2. Replant the parent clump immediately, then plant the individual root sections in a small pot with compost and vermiculite, ensuring the root segment's cut end (crown side) is facing up. The root segment should be entirely buried, but the cut tip should barely be under the surface.
3. Overwinter the pots in a sheltered location. In spring, the root segments should begin to generate new roots, which you will see poking out of the bottom of the pot.
4. When a network of roots is established, and green shoots emerge above the soil line, you can transplant your new sea holly plant into the garden.

Planting Sea Holly From Seed

Most sea holly varieties can be started from seed, though the process has a few steps. The seeds will do best if stratified (chilled to stimulate the growth cycle) first. The easiest method of planting is to sow the seeds directly in the fall, then patiently wait to see what sprouts up in the spring.

You can start sea holly seeds indoors if you want a head start.

1. After collecting seeds in the fall, chill them for about four weeks in the refrigerator.
2. Plant them in small pots filled with a sandy potting mix.
3. Germination can take one week to a month. Germination can take up to 90 days when direct sown, considering the winter season.
4. The seedlings can be planted outdoors the following spring when all danger of frost has passed. As with many perennials, these seedlings may take most of the first growing season to establish good root systems—in subsequent years, you can expect them to flower at the expected time.

Potting and Repotting Sea Holly

Sea holly is rarely grown as a potted plant on decks or patios since it does not like the rich fertility of most potting mixes. It does much better in dry, sandy, even barren soils.

You can grow sea holly in a well-draining container filled with a sandy potting mix, such as a 50-50 mixture of standard potting soil and succulent/cactus mix. Do not overwater potted plants; ensure they get as much direct sunlight as possible.

Potted sea holly plants will need to be moved to a sheltered location for the winter, such as tucked in beside a garden wall or placed in a cold frame. But don't try to move them indoors as houseplants, as sea holly needs a winter dormancy period brought on by cool conditions.

Overwintering

Provide protection against winter cold to allow the rosettes to overwinter successfully. Otherwise, the plant will use all of its energy to make new foliage in the spring. Lay evergreen branches over the plants or mulch with pine needles.

Withhold water in the fall, as it does not care for damp winter soil. After flowering is complete, cut the flower stalks and foliage back to just above ground level. You can also leave the flowers for winter interest.

Common Pests & Plant Diseases

Sea holly is generally free of nearly all pests and diseases in dry soils. However, damp soil can be susceptible to fungal leaf spot diseases and root rot. Powdery mildew can be problematic; base watering and good air circulation will minimize mildew infection. Moist conditions may also encourage aphids, slugs, and snails to feed on the foliage.

Fungicides can help control serious leaf spot disease. Aphids can usually be easily controlled with horticultural oil (or ignored since the damage is rarely severe). Slugs and snails can be removed by hand or controlled with baits.

How to Get Sea Holly to Bloom

Bloom Months

Sea holly typically blooms from mid-summer to fall. You can plant them in the hottest, sunniest part of your garden, and they’ll thrive, even if they’re mostly left alone.

What Do Sea Holly Flowers Look and Smell Like?

Native to Europe, sea holly flowers have green or blue cones and a distinctive bract collar in silver, white, green, or bluish-purple. The colors have a painted-on, metallic sheen that changes in the sunlight, and their stems can be green or silvery blue, depending on the variety.

The dark green sea holly leaves form a rosette at or near the plant's base. They are long and narrow, slightly heart-shaped with toothed edges, and spiny.

Sea holly flowers have a scent that attracts butterflies and birds, although some report some species have a scent that is hardly noticeable to humans and may not be a nice scent.

How to Encourage More Blooms

When grown in properly dry soil and sunny conditions, there's rarely any problem getting sea holly to bloom in its typical summer-to-fall flowering period.

Ensure it has full sun for at least eight hours to encourage more blooms. Also, avoid giving fertilizer or planting it in an area with rich, fertile soil. Poor soil usually yields better flowering. Also, remove dead flower heads to encourage more blooms.

Common Problems With Sea Holly

Sea holly is mainly free of complaints, but you may find that the plants can become overly leggy sometimes.

Leggy Stems

These plants can become sprawling if they are planted in soil that is too fertile and rich. Plant these plants in dry, sandy soil where other plants won't grow. Such sprawling may also occur if the sea holly doesn't get enough sun or if you overwater it.